It is known that, by adding sintering aids to the Si starting powder and by carrying out sintering, following nitridation, at temperatures above approximately 1600.degree. C. with or without the application of pressure, it is possible to obtain reaction-bonded Si.sub.3 N.sub.4 articles having densities of more than 95% TD. This process has the advantage, as compared with the direct sintering of Si.sub.3 N.sub.4 powder with the addition of sintering aids, that during the temperature treatment of the pre-shaped articles, no shrinkage results from the nitridation in an atmosphere containing nitrogen and only slight shrinkage results from the post-sintering. As a result, it is possible to manufacture sintered shaped articles with great precision as to size, such precision usually being achieved only by mechanical finishing. Because of the hardness of the material, finishing of Si.sub.3 N.sub.4 is very expensive and possible only with diamond tools (cf. DE-C2-28 55 785, DE-A-29 10 943, U.S. Pat. No. 4,285,895 and J. A. Mangels in "Ceramic Bulletin", vol. 60, No. 12 (1981), pages 1306-1310).
The strongly exothermic nitridation reaction itself is customarily carried out in several stages, whether or not sintering auxiliaries are used in the starting mixture. During the reaction, at temperatures below the melting point of silicon (1410.degree. C.), one or more dwell times are introduced in order to avoid premature melting of the silicon. The last nitridation stage can be carried out either below or above the melting point of silicon. In all cases, however, this heating in stages results in a total nitridation time, especially in the case of relatively large shaped articles and relatively large charges, of more than 100 hours.
The admixture of hydrogen with the nitrogen gas or the presence of iron in the starting powder accelerates the nitridation reaction, thereby making it possible to reduce the total nitridation time a little. The effect of these measures and other parameters, such as purity and fineness of the starting powder, on the formation and the characteristics of reaction-bonded silicon nitride is described in detail in the review by A. J. Moulson in "Journal of Materials Science" 14 (1979), pages 1017-1051.
By using an atmosphere containing nitrogen to which hydrogen and an inert gas have been added and by controlling the temperature in relation to the gas consumption during the nitridation, it is possible to improve the characteristics of reaction-bonded silicon nitride. In this case also, however, more than 100 hours are required to obtain a high degree of nitridation (cf. J. A. Mangels in "Ceramic Bulletin", vol. 60, No. 6 (1981), pages 613-617).
By using especially fine and pure silicon starting powders, optionally in admixture with silicon nitride powders, and by pre-compacting the pre-shaped silicon article by sintering at temperatures below the melting point of silicon, it is disclosed that the nitridation time can be reduced (cf. GB-PS-15 46 928). It is stated in that specification that the time required to obtain a degree of nitridation of approximately 80% can be reduced to less than a day by applying a nitrogen overpressure of up to approximately 50 atmospheres (about 5 MPa). As is apparent from the Examples, the best results, however, were achieved at atmosphereic pressure in a stream of nitrogen and with the nitridation being continued for 100 hours.
The result of reaction-sintering silicon nitride at a nitrogen pressure of at most 100 MPa and at a temperature of at most 2000.degree. C. has been examined in a study by J. A. Kryl in "Pri-vo i Primenenie Sverkhtverd, Materialov", Kiev 1983, pages 15-20 (ref. C.A. vol. 100: 179168 w). In that study it was found that, at a heating rate of less than 20.degree. per minute, a nitrogen pressure of 10 MPa and with an isothermic dwell time of 4 hours, a degree of nitridation of 85.6% was achieved. At a nitrogen pressure of 100 MPa it was possible to produce specimen articles having a density of from 80 to 85% TD.
Similarly, T. Fujikawa et al. reported at the "International Symposium on Ceramic Components for Engines", Oct. 17-21, 1983, Hakone, Japan, on preliminary tests according to which, using a nitrogen pressure of approximately 500 Kgf/cm.sup.2 (about 50 MPa), it was possible to obtain from silicon powder or from silicon powder in admixture with Si.sub.3 N.sub.4 powder (50:50), reaction-sintered Si.sub.3 N.sub.4 shaped articles having a density of at most 84% TD.
Although it is known that in processes for the manufacture of shaped articles of reaction-bonded silicon nitride by nitridation of pre-shaped articles of silicon powder in several stages using nitrogen under elevated pressure, the nitridation can be accelerated, the problem of achieving a reduction in the total nitridation time to less than 15 hours and increasing the conversion of silicon to silicon nitride to at least 95% has hitherto remained unsolved.